To date, carbon fibers have been used in various composite materials because of their good mechanical properties, high electrical conductivity, high thermal conductivity, etc.
Recently, higher functionalities have come to be required for various materials. Additives which can improve physical properties, such as electrical, mechanical, or thermal properties, of a matrix comprised of solid materials, such as resin, ceramics, and metal, without damaging the characteristics of the matrix have been sought after. Additionally, additives which can improve physical properties of liquids, such as fuels, oil, and lubricants have also been sought.
By the way, regarding the carbon fiber, fine carbon fibers, such as carbon nano structures exemplified by the carbon nanotube (hereinafter, referred to also as “CNT”), have been attracting public attention in various fields.
The graphite layers that make up the carbon nano structures are materials normally comprised of regular arrays of six-membered rings whose structures can bring about specific electrical properties, as well as chemically, mechanically, and thermally stable properties. As long as such fine carbon fibers can retain such properties upon combining and dispersing into solid materials, including various resins, ceramics, metals, etc., or into liquid materials, including fuels, lubricant agents, etc., their usefulness as additives for improving material properties can be expected.
To date, tasks of developing a mass production of the CNTs or fine carbon fibers, enhancing their purity, and developing their separation and purifying techniques have been regarded as important, and many surface improvements and dispersion techniques, and application developments have been proposed. However, the most important theme of technical development for the CNTs is the technique of mass production of CNTs which can disperse easily and uniformly in a composite material. The reason that it has been not attained satisfactorily is ascribable to the diversity of the structures of the CNTs and fine carbon fibers.
As being different from normal molecules, the CNTs possess specific characteristic variables such as thickness (outside diameter), length, chirality, and spatial construction, and it is conceivable that these variables can be controlled at the synthetic stage of CNTs. However, such controls in the synthesis for CNTs of under nano levels have not been sufficiently attained yet. Typical reports for CNTs and fine carbon fibers will be described below.
Patent Literature 1 discloses a resin composition comprising agglomerates wherein each of the agglomerate is composed of mutually entangled carbon fibrils having 3.5-70 nm in diameter, and wherein the agglomerates possess a diameter in the range of 0.10 to 0.25 mm with a maximum diameter of not more than 0.25 mm. It is noted that the numeric data such as the maximum diameter, diameter, etc., for the carbon fibril agglomerates are those measured prior to combining with a resin, as is clear from the descriptions in the examples and other parts of the Patent Literature 1.
Patent Literature 2 discloses a composite material where a carbon fibrous material is added to the matrix, the carbon fibrous material mainly comprising agglomerates each of which is composed of carbon fibers having 50-5000 nm in diameter, the mutual contacting points among the carbon fibers being fixed with carbonized carbonaceous substance, and each agglomerate having a size of 5 μm-500 μm. In the Patent Literature 2, the numeric data such as the size of the agglomerate, etc., are those measured prior to the combining into resin, too.
Using carbon fibrous agglomerates such as described above, it is expected that the dispersibility of carbon nano structures within a resin matrix will improve to a certain degree as compared to that of using bigger lumps of carbon fibers. The agglomerates prepared by dispersing carbon fibrils under a certain shearing force, such as in a vibrating ball mill or the like according to the Patent Literature 1, however, have relatively high bulk densities. Thus, they do not fulfill the need for ideal additives that is capable of improving various characteristics, such as electric conductivity, of a matrix effectively at low dosages.
With respect to the carbon fibrous agglomerates disclosed in the Patent Literature 2, it is necessary to provide an additional step for fixing carbon fibers at their mutual contacting points after synthesis of the carbon fibers, and thus the efficiency of manufacturing becomes worse. Further, since the carbon fibrous agglomerate is manufactured by heating carbon fibers in a state such that mutual contacting points among the carbon fibers are formed by compression molding after synthesis of the carbon fibers, and wherein fixing of fibers at the contacting points is done by carbonization of organic residues primarily attached to the surface of the carbon fibers, or carbonization of an organic compound additionally added as a binder, the affixing forces at the contacting points are weak. In addition, the electrical properties of the carbon fibrous agglomerate per se are not well, although a certain degree of improvement in the electrical properties would be expected as compared with the case of pulverized monofibrous carbon fibers. Thus, when these carbon fibrous agglomerates are added to a matrix such as a resin, the carbon fibers fixed at the contacting points are easily detached from each other, and the carbon fibrous agglomerates are no longer maintained in the matrix. Therefore, it is difficult to construct preferable conductive paths in a matrix such that good electrical properties may be conferred on the matrix by a small additive amount of the fibrous agglomerates. Furthermore, when a binder is added to promote fixing and carbonization at the contacting points, fibers in the obtained fibrous agglomerates would have large diameters and inferior surface characteristics because the added binder is attached to the whole surface area of the fibers rather than to a limited area on the contacting points.
Further, in Patent Literature 3, the disclosed is vapor phase method's carbon fibers which is obtained by using the vapor phase method for producing carbon fibers wherein raw material for the carbon fiber, catalyst, etc., are injected toward the inner wall of a reaction chamber, and undergo reaction, and which are characterized in that the mean fibrous diameter is in the range of 80-500 nm, and more than 65% of all fibers are involved in the range of the mean fibrous diameter ±20%. However, since a raw material supplying part used in the vapor phase method adapts a way of activating the reaction by injecting the catalyst which is utilized in the early stage of carbon fiber synthesis toward the inner wall of the reaction chamber, and thus the catalyst is produced by coming into collision with the inner wall of the reaction chamber, it is considered that the turbulence of flows at the region of the catalyst production and the neighborhood of the region is very large. Therefore, the distribution of size of the produced catalysts becomes broader. Thus, it is difficult to produce carbon fibers having a more sharp distribution of the diameter. In addition, since the method where the catalyst comes into collision with the inner wall of the reaction chamber uses only a part of the surface area of the inner surface of the reaction chamber, and it does not use the internal space of the reaction chamber, it will not be suitable to a more developed mass production of carbon fibers.
Patent Literature 4 provides a carbon fibrous structure which has a three dimensional network shape and comprises carbon fibers of 15-100 nm in the outer diameter and which is produced by a vapor phase method. However, the distribution of the fiber outer diameter of the carbon fibrous structure is broad. Thus, when the carbon fibrous structures are added to a resin in order to give the electrical conductivity, or when a composite product is manufactured by the injection molding process, a fear that the skin layer of the product becomes thicker will arise as described below, and the variation in the data of electrical conductivity for the products will be large. Therefore, the development of carbon fibrous aggregate having a sharp distribution of outer diameters of carbon fibers is urgently necessary.
In Patent Literature 5, there are a description about branched carbon fiber, and a description that the reaction should be performed under the condition of using ferric catalyst such as ferrocene at an extreme high concentration in order to enhance the branching degree. However, in this process, since the reaction is activated by injecting the catalyst which is utilized in the early stage of carbon fiber synthesis toward the inner wall of the reaction chamber in order to produce the branched carbon fiber, and thus the same reasons as described above go for this process, it is expected that the production of carbon fibers having a more sharp distribution of the diameter is difficult.
In Patent Literature 6, a dendritic form of fine carbon fibers which bifurcates into many branches and which is produced by the polymer particles' method is disclosed. However, since the method described in this literature depends on charring of the polymer, it is not suitable to the mass production.